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A study of the spectra of pyrene Vilkos, Verna Vilma Beruthe


The polarized absorption and fluorescence spectra of pyrene-h₁₀ and of pyrene-d₁₀ as guests in biphenyl and fluorene single crystal matrices at about 10°K have been measured. Incomplete vibrational analyses of the ground and of the first two excited singlet electronic states are given in terms of these spectra and further information is obtained from low-temperature spectra in polycrystalline n-paraffin samples. Ground state vibrational data were also obtained from laser-Raman studies of a pyrene-h₁₀ single crystal and of pyrene-h₁₀ and -d₁₀ in solutions of benzene, carbon disulphide and carbon tetrachloride; as well, the fundamental frequencies of the two isotopic species were calculated using approximate force fields that have been applied successfully to similar molecules. The spectra are complex. In fluorescence, an exceptionally large number of modes appear which could not be accounted for in terms of a[sub g] and b[sub 3g] fundamentals. These lines either (a) are common to all matrices used and appear through Fermi-resonance or are either Franck-Condon or Herzberg-Teller allowed, or (b) appear in only one or two matrices being induced by the crystal forces of the host environment. The effect of Herzberg-Teller coupling is the introduction of intensity which does not arise from the ¹B[sub 2u] → ¹A[sub g] transition; in contrast, the remaining interactions are similar in that they cause only a redistribution of the intensity of the pure electronic transition amongst various vibronic bands. Mechanical coupling in the lowered symmetry of the site in the host lattice causes a significant redefinition of the atomic displacements in the pyrene guest molecule and plays an unusually important part in these spectra. For example, the intensities of many lines assigned as combination bands were inconsistent with the Franck-Condon predictions. Moreover, the sheer numbers of lines appearing in the fundamental region in one matrix or another suggest that almost all fundamentals may be present and that pyrene in its first excited electronic state is probably not situated at the inversion and mirror-plane sites in the biphenyl and fluorene lattices, respectively. Further, the pyrene molecule has probably caused a localized lattice distortion; a measurement of the absolute intensity of absorption indicates that the unexcited guest molecule causes a local expansion of the fluorene lattice in the ab plane. An upper estimate for these matrix induced interactions is about 10 cm⁻¹ and probably arises from repulsive contributions, since for all the matrices used in this work, pyrene is somewhat bigger (usually wider) than the host molecule it replaces. There is no mirror-image relationship between fluorescence and absorption and this is probably due to changes primarily in the off-diagonal elements of the force constant matrix. The first transition is assigned ¹B[sub 2u] ← ¹A[sub g] (short- axis polarized) and the second ¹B[sub 1u] ← ¹A[sub g] (long-axis polarized). Vibrations belonging to the first excited electronic state and having b[sub 3g] total symmetry provide a vibronic interaction with the second electronic state. The more nearly degenerate the interacting states become the more intense is the coupling and in the single crystal spectra where "solvent" shifts move the two excited electronic states close together in energy the usual Herzberg-Teller approach does not apply and vibronic basis functions must be considered. Thus, a[sub g] fundamentals of the first electronic state which are very weak or g absent in the b-polarized spectrum appear in c' polarization built on false origins (the more important being a b[sub 3g] combination at 1423 cm⁻¹ and a b[sub 3g] fundamental at about 1500 cm⁻¹ in pyrene-h₁₀ in biphenyl) in the energy region of the second electronic system. In the n-paraffin spectra, the combination corresponding to 1423 cm⁻¹ could not be found while the fundamental was easily located (at 1564 cm⁻¹ in n-heptane). Thus, while the changes in shape of the pyrene molecule in going from the ground to the two excited states are different, the sets of normal coordinates belonging to the first and second excited electronic states are probably nearly parallel to each other but not to the ground state set.

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